[llvm] [AMDGPU] Set MaxAtomicSizeInBitsSupported. (PR #75185)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: James Y Knight (jyknight)

<details>
<summary>Changes</summary>

This will result in larger atomic operations getting expanded to `__atomic_*` libcalls via AtomicExpandPass, which matches what Clang already does in the frontend.

While AMDGPU currently disables the use of all libcalls, I've changed it to instead disable all of them _except_ the atomic ones. Those are already be emitted by the Clang frontend, and enabling them in the backend allows the same behavior there.

---
Full diff: https://github.com/llvm/llvm-project/pull/75185.diff


2 Files Affected:

- (modified) llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp (+7-3) 
- (added) llvm/test/CodeGen/AMDGPU/atomic-oversize.ll (+10) 


``````````diff
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
index fcbdf51b03c1f..78092675057df 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelLowering.cpp
@@ -506,9 +506,11 @@ AMDGPUTargetLowering::AMDGPUTargetLowering(const TargetMachine &TM,
   setOperationAction(ISD::SELECT, MVT::v12f32, Promote);
   AddPromotedToType(ISD::SELECT, MVT::v12f32, MVT::v12i32);
 
-  // There are no libcalls of any kind.
-  for (int I = 0; I < RTLIB::UNKNOWN_LIBCALL; ++I)
-    setLibcallName(static_cast<RTLIB::Libcall>(I), nullptr);
+  // Disable most libcalls.
+  for (int I = 0; I < RTLIB::UNKNOWN_LIBCALL; ++I) {
+    if (I < RTLIB::ATOMIC_LOAD || I > RTLIB::ATOMIC_FETCH_NAND_16)
+      setLibcallName(static_cast<RTLIB::Libcall>(I), nullptr);
+  }
 
   setSchedulingPreference(Sched::RegPressure);
   setJumpIsExpensive(true);
@@ -556,6 +558,8 @@ AMDGPUTargetLowering::AMDGPUTargetLowering(const TargetMachine &TM,
                        ISD::FSUB,       ISD::FNEG,
                        ISD::FABS,       ISD::AssertZext,
                        ISD::AssertSext, ISD::INTRINSIC_WO_CHAIN});
+
+  setMaxAtomicSizeInBitsSupported(64);
 }
 
 bool AMDGPUTargetLowering::mayIgnoreSignedZero(SDValue Op) const {
diff --git a/llvm/test/CodeGen/AMDGPU/atomic-oversize.ll b/llvm/test/CodeGen/AMDGPU/atomic-oversize.ll
new file mode 100644
index 0000000000000..f62a93f523365
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/atomic-oversize.ll
@@ -0,0 +1,10 @@
+; RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -verify-machineinstrs < %s | FileCheck %s
+
+define void @test(ptr %a) nounwind {
+; CHECK-LABEL: test:
+; CHECK: __atomic_load_16
+; CHECK: __atomic_store_16
+  %1 = load atomic i128, ptr %a seq_cst, align 16
+  store atomic i128 %1, ptr %a seq_cst, align 16
+  ret void
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/75185